Autosomal dominant polycystic kidney disease (ADPKD) is characterized by kidney cyst formation that progresses to end-stage renal failure at a high frequency. The disease also encompasses systemic vascular complications such as hypertension and vascular aneurysms that increase the overall mortality of the disease. Mutations in two genes Pkd1 and Pkd2 account for most cases of ADPKD. Recent studies show that PKD2 functions as a calcium-activated, calcium-permeable cation channel;and PKD1 plays a role in regulating the PKD2 channel activity. A high concentration of PKD1 and PKD2 co-localizes on primary cilia of renal epithelia. The current working model is that cilium-associated PKD1/PKD2 protein complex mediates Ca2+ influx through cilia bending in response to fluid flow in the renal tubules. Loss of either PKD1 or PK02 leads to a common defect in ciliary Ca2+ influx and this is one of the contributing factors of cyst formation in the kidney. This proposal utilizes a novel ciliary model of PKD2 in Drosophila sperm to substantiate the current model of PKD2 as cilium-associated, mechano-/chemo-sensitive cation channel. We show that Drosophila PKD2 (CG6504) is a conserved cation channel of the PKD2 family that localizes to sperm flagellar cilia where it is required for responding to signals that regulate asymmetric flagellar motility and directional sperm movement. This Drosophila PKD2 function appears to be analogous to mammalian PKD2 function in the motile nodal cilia. Directional beating of nodal cilia, which contain PKD2, is required for left-right axis determination in mammals. The goals of this proposal are to elucidate PKD2 channel regulation and signaling through structure-function analyses of the Drosophila PKD2 protein, phosphoproteomic analyses of PKD2-dependent downstream protein phosphorylation. and genetic analyses of candidate kinases and phosphoproteins in the pathway. The proposed research will identify structural elements involved in PKD2 channel regulation and function in vivo in a ciliary context, and will also identify PKD2-dependent downstream phosphoproteins and use them to delineate a genetic pathway for PKD2 signaling and function.